A barrier which separates the mundane from the Transcendent Reality. A true Wonderwall will always have a crack, or a slit or an opening which allows anyone a glimpse of what lies beyond the Wonderwall.

I peeked through the opening of the Wonderwall and saw a dim glimmer of Krishna.

Something (most typically a song) that has been so over used that it is boring and painful to hear. No matter how brilliant you thought it was when you first heard it after the millionth time today on the same radio show it is now annoying.

Named after the Oasis song of the same name which, despite being brilliant in many ways, has been rejected by many fans as "a bit wonderwall" due to over playing on air.

Person 1: "Have you heard that new Blur song?"

Person 2: "Only a million times already today... I did like it but now it's abit Wonderwall"

Wall- Any of various permanent upright constructions having a length much greater than the thickness and presenting a continuous surface except where pierced by doors, windows, etc.: used for shelter, protection, or privacy, or to subdivide interior space, to support floors, roofs, or the like, to retain earth, to fence in an area, etc.

So a wonderwall can be widely considered as a noun that is a strong foundation full of admiration.